The present invention relates, in general, to optical coupling of integrated circuits and, more particularly, to the transfer of data between integrated circuits by means of modulated light beams directed by holograms into planar optical waveguides.
As the density of electronic integrated circuits increased, the limiting factor for circuit speed increasingly became propagation delay due to capacitance associated with circuit interconnection. Traditionally, interconnection within an integrated circuit was accomplished by etched runs of metal such as aluminum or aluminum alloys. As circuit density increased, the spacing between adjacent metal runs was reduced. The width of these metal runs was also reduced. Since the resistance of a material is related to its cross-sectional area, reducing the width of the metal runs caused resistance to increase. This was countered by making the metal runs taller, increasing cross sectional area and thus reducing resistance. The penalty paid was a net increase in parasitic capacitance between metal runs. Not only were the metal runs closer together, which increased parasitic capacitive coupling, but the facing edges were of larger area, further increasing capacitive coupling, which in turn increased capacitive loading. At relatively low clock speeds, the capacitive loading was not a significant factor. As newer applications began to push clock speeds into the one hundred megahertz range and beyond, capacitive loading became a limiting factor for circuit performance by limiting circuit speed and increasing circuit cross talk.
Communication between multiple integrated circuits also had speed limitations due to interconnection. On printed circuit boards, the power, and the associated delay, required to drive the line capacitance generated between interconnection leads and the ground plane, became significant in the desired high frequency range. Additionally, wide data busses were plagued by a phenomenon known as simultaneous switching noise. Simultaneous switching noise was related to parasitic inductances associated with power and ground interconnections between the chip and the substrate. The level of the simultaneous switching noise was determined by the parasitic inductance, the width of the bus, and the rate of change of the drive current that charged up the interconnect lines. Finally, at these frequencies, impedance matching became necessary to decrease signal settling time, thus adversely affecting the efficiency of data transfer and increasing power dissipation.
In order to overcome the limitations imposed by circuit interconnection, the use of optical interconnection was explored. A number of optical interconnect approaches were advanced by Goodman, et. al., "Optical Interconnections for VLSI Systems", Proceedings of IEEE, vol 72, No.7, July 1984. One approach consisted of a number of opto-electronic transmitters, normally lasers, placed near the edge of an integrated circuit. The opto-electronic transmitters aimed beams of light at a holographic routing element located above the integrated circuit. The beams of light were modulated such that the beams of light contained the data to be transferred. The holographic routing element diffracted the beams of light back to opto-electronic receivers on the surface of the integrated circuit.
Another approach, more typically used for a clock signal as opposed to data transfer, consisted of an opto-electronic transmitter located above an integrated circuit. Between the opto-electronic transmitter and the integrated circuit was located a holographic routing element. The opto-electronic transmitter emitted a modulated signal which was aimed by the holographic routing element onto opto-electronic receivers on the surface of the integrated circuit.
Still another approach utilized optical waveguides. Of particular interest were planar optical waveguides. In this approach, the opto-electronic transmitters and receivers were located as in either of the first two approaches described above. The optical signals, instead of being diffracted directly to the opto-electronic receivers, were diffracted by holographic elements into a planar optical waveguide. The signals then propagated from one point to another through the planar optical waveguide before being diffracted out of the planar optical waveguide by holographic elements and focused upon opto-electronic receivers on the surface of an integrated circuit.
In these approaches toward optical interconnection, the holographic routing elements had to be precisely aligned with the opto-electronic receivers of the integrated circuits. The opto-electronic transmitters or other light sources then had to be precisely aligned relative to the holographic routing elements in order for the modulated light beams emitted by the sources to be directed by the holographic routing element to the proper opto-electronic receivers. Achieving the required precision of alignment made assembly into a package extremely challenging. The kinds of tolerances required were normally associated with device fabrication processes rather than with package assembly. Also, the physical arrangement of the various elements made the package rather large and bulky.
Fabrication requirements of opto-electronic transmitters and receivers further limited the utility of these approaches. The opto-electronic devices were typically fabricated from III-V materials such as gallium arsenide or indium phosphide. These materials emit light in the near infrared light spectrum. Therefore, the integrated circuits which were to be interconnected were either manufactured on III-V substrates, or a III-V epitaxial layer structure had to be grown on a silicon substrate. The former option severely limited the number of applications in which optical interconnection could be used due to the pervasive position of silicon in the semiconductor industry. The latter approach presented particularly challenging obstacles. It was extremely difficult to grow a III-V epitaxial layer structure upon a silicon substrate with a crystalline defect density low enough to provide for the fabrication of reliable opto-electronic devices.